This invention relates to radiation-curable organopolysiloxane compositions, to their use in a coating method and to an article produced by said method. In one aspect this invention relates to radiation-curable, mercaptopropyl-containing polydiorganosiloxane compositions which have an improved curing odor, to a method of coating and curing said composition onto a substrate and to a coated substrate obtained therefrom.
In the pressure sensitive adhesive art it is often desirable to prepare an adhesive-bearing article such as a label or a tape and to store the article for some time before use. In the storing process the adhesive-bearing article is conveniently adhered to an intermediate surface from which it must be easily removed before use. For example, said tape may be wound in a coil whereby the adhesive surface of one turn of the tape is adhered to the non-adhesive surface of the previous turn of the tape. Preferential release of the upper turn of tape with its adhesive from the previous turn is needed to unwind and use the tape. In another example, adhesive-bearing labels are frequently supplied with a protective paper over the adhesive, which paper must be removable from the adhesive before the label is applied in its intended use.
Polyorganosiloxanes have been used for a long time to coat solid substrates, such as paper, to improve the release of adhesive materials therefrom. As the adhesive properties and holding power of adhesive materials have improved, the need for improved release coatings has increased. Improved release coatings have also been required by other factors such as increasing environmental concerns, a desire for faster coating and curing processes, and a need to lower process energy costs. Solventless, radiation-curable coatings have thus been sought to meet these needs.
Radiation-curable organopolysiloxane compositions bearing mercaptoorgano radicals are known broadly from Warrick, Canadian Pat. No. 653,301; Berger, et al., U.S. Pat. No. 3,726,710 and Viventi, U.S. Pat. No. 3,816,282; however, it is the disclosures of Michael, et al. U.S. Pat. No. 3,873,499; Gant, British Pat. No. 1,409,223; and Bokerman, et al. U.S. Pat. No. 4,052,529 relating to radiation-curable coating compositions comprising an organopolysiloxane bearing mercaptoorgano radicals and a polymethylvinylsiloxane curing agent that are of interest. Because of the presence of a curing odor, which develops during ultraviolet light curing in air, said compositions comprising a mercaptopropyl-substituted polydiorganosiloxane and a polymethylvinylsiloxane curing agent have met some resistance to being accepted in the marketplace.
By curing odor it is meant herein an odor which emanates from the cured composition after the radiation-curable composition has been applied to a substrate and has been exposed to ultraviolet light in an oxygen-containing atmosphere, such as air, to effect the curing process. This curing odor, which may persist for minutes or hours after the curing is completed, has been described by some observers as a sour odor or as a mercaptan-like odor. This odor is very noticable, especially when the radiation-curable composition is used in thin layer applications, such as in a paper-coating process.